/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.support;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Member;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Collections;
import java.util.Map;
import java.util.Set;

import org.springframework.core.convert.ConversionFailedException;
import org.springframework.core.convert.TypeDescriptor;
import org.springframework.core.convert.converter.ConditionalGenericConverter;
import org.springframework.lang.Nullable;
import org.springframework.util.ClassUtils;
import org.springframework.util.ConcurrentReferenceHashMap;
import org.springframework.util.ReflectionUtils;

/**
 * Generic converter that uses conventions to convert a source object to a
 * {@code targetType} by delegating to a method on the source object or to
 * a static factory method or constructor on the {@code targetType}.
 *
 * <h3>Conversion Algorithm</h3>
 * <ol>
 * <li>Invoke a non-static {@code to[targetType.simpleName]()} method on the
 * source object that has a return type equal to {@code targetType}, if such
 * a method exists. For example, {@code org.example.Bar Foo#toBar()} is a
 * method that follows this convention.
 * <li>Otherwise invoke a <em>static</em> {@code valueOf(sourceType)} or Java
 * 8 style <em>static</em> {@code of(sourceType)} or {@code from(sourceType)}
 * method on the {@code targetType}, if such a method exists.
 * <li>Otherwise invoke a constructor on the {@code targetType} that accepts
 * a single {@code sourceType} argument, if such a constructor exists.
 * <li>Otherwise throw a {@link ConversionFailedException}.
 * </ol>
 *
 * <p><strong>Warning</strong>: this converter does <em>not</em> support the
 * {@link Object#toString()} method for converting from a {@code sourceType}
 * to {@code java.lang.String}. For {@code toString()} support, use
 * {@link FallbackObjectToStringConverter} instead.
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @see FallbackObjectToStringConverter
 * @since 3.0
 */
final class ObjectToObjectConverter implements ConditionalGenericConverter {

    // Cache for the latest to-method resolved on a given Class
    private static final Map<Class<?>, Member> conversionMemberCache =
            new ConcurrentReferenceHashMap<>(32);


    @Override
    public Set<ConvertiblePair> getConvertibleTypes() {
        return Collections.singleton(new ConvertiblePair(Object.class, Object.class));
    }

    @Override
    public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
        return (sourceType.getType() != targetType.getType() &&
                hasConversionMethodOrConstructor(targetType.getType(), sourceType.getType()));
    }

    @Override
    @Nullable
    public Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
        if (source == null) {
            return null;
        }
        Class<?> sourceClass = sourceType.getType();
        Class<?> targetClass = targetType.getType();
        Member member = getValidatedMember(targetClass, sourceClass);

        try {
            if (member instanceof Method) {
                Method method = (Method) member;
                ReflectionUtils.makeAccessible(method);
                if (!Modifier.isStatic(method.getModifiers())) {
                    return method.invoke(source);
                } else {
                    return method.invoke(null, source);
                }
            } else if (member instanceof Constructor) {
                Constructor<?> ctor = (Constructor<?>) member;
                ReflectionUtils.makeAccessible(ctor);
                return ctor.newInstance(source);
            }
        } catch (InvocationTargetException ex) {
            throw new ConversionFailedException(sourceType, targetType, source, ex.getTargetException());
        } catch (Throwable ex) {
            throw new ConversionFailedException(sourceType, targetType, source, ex);
        }

        // If sourceClass is Number and targetClass is Integer, the following message should expand to:
        // No toInteger() method exists on java.lang.Number, and no static valueOf/of/from(java.lang.Number)
        // method or Integer(java.lang.Number) constructor exists on java.lang.Integer.
        throw new IllegalStateException(String.format("No to%3$s() method exists on %1$s, " +
                        "and no static valueOf/of/from(%1$s) method or %3$s(%1$s) constructor exists on %2$s.",
                sourceClass.getName(), targetClass.getName(), targetClass.getSimpleName()));
    }


    static boolean hasConversionMethodOrConstructor(Class<?> targetClass, Class<?> sourceClass) {
        return (getValidatedMember(targetClass, sourceClass) != null);
    }

    @Nullable
    private static Member getValidatedMember(Class<?> targetClass, Class<?> sourceClass) {
        Member member = conversionMemberCache.get(targetClass);
        if (isApplicable(member, sourceClass)) {
            return member;
        }

        member = determineToMethod(targetClass, sourceClass);
        if (member == null) {
            member = determineFactoryMethod(targetClass, sourceClass);
            if (member == null) {
                member = determineFactoryConstructor(targetClass, sourceClass);
                if (member == null) {
                    return null;
                }
            }
        }

        conversionMemberCache.put(targetClass, member);
        return member;
    }

    private static boolean isApplicable(Member member, Class<?> sourceClass) {
        if (member instanceof Method) {
            Method method = (Method) member;
            return (!Modifier.isStatic(method.getModifiers()) ?
                    ClassUtils.isAssignable(method.getDeclaringClass(), sourceClass) :
                    method.getParameterTypes()[0] == sourceClass);
        } else if (member instanceof Constructor) {
            Constructor<?> ctor = (Constructor<?>) member;
            return (ctor.getParameterTypes()[0] == sourceClass);
        } else {
            return false;
        }
    }

    @Nullable
    private static Method determineToMethod(Class<?> targetClass, Class<?> sourceClass) {
        if (String.class == targetClass || String.class == sourceClass) {
            // Do not accept a toString() method or any to methods on String itself
            return null;
        }

        Method method = ClassUtils.getMethodIfAvailable(sourceClass, "to" + targetClass.getSimpleName());
        return (method != null && !Modifier.isStatic(method.getModifiers()) &&
                ClassUtils.isAssignable(targetClass, method.getReturnType()) ? method : null);
    }

    @Nullable
    private static Method determineFactoryMethod(Class<?> targetClass, Class<?> sourceClass) {
        if (String.class == targetClass) {
            // Do not accept the String.valueOf(Object) method
            return null;
        }

        Method method = ClassUtils.getStaticMethod(targetClass, "valueOf", sourceClass);
        if (method == null) {
            method = ClassUtils.getStaticMethod(targetClass, "of", sourceClass);
            if (method == null) {
                method = ClassUtils.getStaticMethod(targetClass, "from", sourceClass);
            }
        }
        return method;
    }

    @Nullable
    private static Constructor<?> determineFactoryConstructor(Class<?> targetClass, Class<?> sourceClass) {
        return ClassUtils.getConstructorIfAvailable(targetClass, sourceClass);
    }

}
